Flat Feet

This is a bit of a stab in the dark, but I've got this problem with my foot.....
I'm a bit flat footed, and made sure I got a good pair of running shoes before starting running again a month ago. But I had a few tweaks (it never actually hurt when I was running) and after a 10k yesterday, the entire side underarch of one of my feet has slightly swollen up and is a bit painful to walk on.
Do I need to buy special insoles? Aren't they very expensive? Do I need to see a sports physio? This is soooooo frustrating, because my running has just started to go well! Advice would be appreciated....
Cat

Can't offer any advice, except to go to a good running shop and let them analyze you. Take an old pair of running shoes as well as the new ones (they will look at wear of old ones and make deductions).